The studies proposed in this application are designed to bridge the gap between the two general approaches to animal feeding behavior. The homeostatic approach emphasizes animals' responses to a variety of internal cues, while the ecological approach has sought explanations for animals' patterns of food intake in external factors, such as foraging cost and food availability. Because these models were developed by and remain largely segregated within different scientific disciplines, little is known about the interaction of physiological and environmental determinants of feeding behavior. A third approach, which emphasizes the hedonic aspects of eating, is also poorly integrated into the homeostatic and ecological models. The general goal of the proposed studies is to combine these areas of investigation, by asking whether psychobiological controls of food intake already identified in free-feeding animals have the same effects when ecological constraints also operate. The specific aims are to compare the responsiveness of foraging and free-feeding animals to a variety of previously identified internal and external signals that initiate meals and cues that maintain feeding, and to compare their ability to adjust protein intake in response to internal changes in nutritional state. Foraging animals' regulation of food intake suggests that the animals' responsiveness to internal signals for meal initiation and termination is modified by the external demand of meal cost. As the cost of obtaining meals increases, foragers eat fewer meals per day and increase average meal size, keeping total intake constant over a range of food-access costs. The proposed studies will combine several well-developed technologies. In the laboratory simulation of foraging, animals will be required to work for access to meals and their behavior will be compared to that of animals given free access to meals. Intragastric infusion techniques will allow the separation of oral and postingestive stimuli for the study of palatability, nutrient density and quality. The examination of physiological signals will make use of on-line blood glucose recording and automated infusions of cholecystokinin, so that freely behaving animals can be studied with minimal disturbance by handling. Exteroceptive stimuli will be used to elicit conditioned meal initiation and to signal impending food restriction. Computerized data collection and control will allow the continuous monitoring of all foraging and feeding behavior. This research, which involves the fields of psychology, biology and nutrition, will provide new information on the interaction of internal physiological cues and external environmental cues in the control of caloric intake and nutrient selection under free-feeding and simulated foraging conditions.